1. Field of the Invention
The present invention relates to the field of filters and notably, but not exclusively, to filters in association with modulators employed in the field of radio and television signal broadcast. In this type of application, the filters concerned by the invention are placed between the modulator circuits and the power circuit that drives the aerial.
2. Prior Art
Efforts in this field are towards totally digital transmission systems for both television and radio. Compared to analog transmission systems, digital technology allows a much denser occupation of the spectrum and a greater immunity to noise and interference problems.
For hertzian wave broadcasting, present day digital audio and video broadcast (respectively DAB and DVB) development programs aim to use the UHF IV and V carrier frequency bands.
The modulation technique envisaged, which is known in itself, is coded orthogonal frequency division multiplexing (COFDM). This protocol is used notably in European standards.
Such a form of modulation is well known, being described among others in patent documents EP-A-0 902 574 and WO-A-98 11698. Only the basic concepts shall be recalled here, with reference to FIG. 1.
This simplified diagram shows the functional units that serve to elaborate a phase quadrature modulated signal from two input signals I and Q. These two signals convey modulated information and have a 90xc2x0 phase difference with respect to each other. The two signals are fed to inputs of respective mixers 2, 4 which also receive signals from a 0xc2x0/90xc2x0 dephaser at the frequency Fo=sin wot. The two respective mixers 2,4 thus supply a digital signal which is fed to respective inputs of an adder circuit 6. The output I(binary) of that circuit is supplied to the input of a digital-to-analog converter 8 to form the modulated signal I(a) to be transmitted.
This signal I(a) is generally a signal that carries a large number of carriers, for example 6800 carriers on a 7.61 MHz band, as shown in FIG. 2. This signal has a central frequency termed Fnum positioned at a frequency on the order of 18 MHz.
In order to provide power amplification to this signal, it is first necessary to transpose the frequency Fnum to a higher frequency in the UHF band.
To do this, the technique presently used involves a two-stage transposition, as shown schematically in FIG. 3. The different points of the circuit shown in FIG. 2 are identified by the letters(a) to (d); the signals at these corresponding points are depicted in FIG. 4, which is a graph showing the frequency along the X-axis and the signal level along the Y-axis.
The signal I(a) with a central frequency Fnum is processed by a classical heterodyne circuit 10 with two transposition stages. The input signal (a) passes through a first mixer circuit 12 where it is mixed with a fixed frequency signal Foll having a higher frequency than Fnum. This mixer circuit 12 produces at the output (b) two spectrums S1 and S2 (FIG. 4) corresponding respectively to the difference and the sum of the mixed frequencies.
These two spectrums are separated by a first bandpass type filter 14 whose output transmits only the spectrum S2 of the upper mixed frequency (C). Because the two spectrums are very close in frequency, this separation calls for a highly selective filter. To this end, a surface acoustic wave device (SAW) is normally used. This spectrum is then produced at the input of a second mixer 16 which also receives as an input a mixing frequency Fol2 having a frequency higher than Fol1. As with the first mixer circuit 12, this second mixer circuit 16 produces two spectrums S3 and S4 corresponding respectively to the difference and the sum of the frequencies in the spectrum delivered by the first filter 14 and the frequency Fol2.
The frequencies of signals Fol1 and Fol2 are chosen such that the upper frequency spectrum S4 of filter 16 corresponds to the chosen frequency band (that is the UHF IV and V bands in the example considered). This spectrum S4 is conserved by eliminating the other using a second filter 18.
In the state of the art, this second filter is fixed in frequency. In other words, it selects just one frequencyxe2x80x94or narrow band of frequenciesxe2x80x94by eliminating all the others. This filter is therefore chosen so as to be tuned to the desired output frequency.
Normally, as the transmitter is of the fixed frequency type, the filter 18 is selected so as to pass the range of frequencies around the carrier corresponding to the transmission channel of the UHF band. It is thus necessary to provide a different fixed filter 18 for each transmission channel.
An object of the present invention is to provide a bandpass filter of variable frequency so that it can adapt to different channels, notably in the 400 MHz to 1 GHz frequency band.
In the example considered, such a filter can be implemented as a replacement for the fixed filter 18 to provide flexibility to the circuit 10 with respect to the different channels which can be used.
To this end, a first object of the present invention is to provide a bandpass filter with adjustable central frequency and operative in the UHF band, characterized in that it comprises a series of cells coupled to each other by coupling capacitors, each cell forming a resonant circuit composed of at least one inductor connected in parallel with at least one variable capacitor.
Advantageously, the coupling capacitors are also variable capacitors.
According to a particularly remarkable characteristic of the invention, the filter can be made substantially symmetrical between its signal input and its signal output.
In a preferred embodiment the cells are four in number.
Preferably, each variable capacitor forming the resonant circuits and each variable coupling capacitor is in the form of at least one electrically controllable variable capacitor.
In this case, it is possible to provide that each electrically controllable variable capacitor is formed by at least one voltage controlled variable capacitance diode.
Preferably, each variable coupling capacitor is formed by a pair of variable capacitor diodes connected head to head.
In order to provide an optimization of the input and output impedance matching characteristics, the filter can comprise an input connected to an intermediate tapping of the inductor of the first cell of the series of cells and an output connected to an intermediate tapping of the inductor of the last cell of the series of cells.
Advantageously, the inductors of the first cell and the last cell have a value different from that of the inductor(s) of the intermediate cell(s), the difference in value enabling to employ a same variable control voltage for controlling on the one hand the electrically controllable capacitors of the resonant circuits formed by the first and last cells and on the other hand the electrically controllable capacitor(s) of the resonant circuit(s) formed by the intermediate cell(s).
In this case, the inductors of the first cell and the last cell preferably have an inductance value greater than that of the inductors of the intermediate cell(s)
For an easier implementation of the filter, it is possible to provide that the electrically controllable variable capacitors respectively forming the coupling between the first cell and the cell adjacent to the latter and the coupling between the last cell and the cell adjacent to the latter have a same capacitance value for a same capacitance control voltage over a determined range of control voltages.
Preferably, each inductor is in the form of a microstrip deposited on an insulating substrate.
A second object of the present invention is to provide a bandpass filtering circuit with adjustable central frequency operational in the UHF band, characterized in that it comprises a filter such as described above and voltage supply means for controlling the central frequency.
Advantageously, the voltage supply means produces a first voltage supplied to the inputs controlling the capacitance value of the capacitors forming the resonant circuits of the respective cells, a second voltage supplied to the inputs controlling the capacitance value of the coupling capacitors forming respectively the coupling between the first cell and the cell adjacent to the latter and the coupling between the last cell and the cell adjacent to the latter, and a third voltage supplied to the input controlling the capacitance value of intermediate coupling capacitor(s).
Advantageously, the third voltage is proportional to the first voltage.
Thus, in the application considered, there is a first transposition which is most generally always carried out at a fixed frequency. However, by virtue of the present invention, the second transposition can be frequency agile, the agility being followed by the adjustable filter of the invention.
There is still a transposition which starts from the base band to the signal having an intermediate frequency around Fol1, which can be fixed and for which standard intermediate frequencies can be found. Filters of different sources are indeed abundant on the market, whether they be surface acoustic wave (SAW) devices or other.
On the other hand, the second transposition is very often carried out using a filter to obtain the frequency of the channel, the latter having a much broader band and having to be able to cover all the band, for example bands IV and V in television, which can occupy one frequency octave. And this is where the second transposition gains in being frequency agile.
However, what up until now prevented frequency agility in systems was precisely the fact that there exists no filter which is frequency agile. Indeed, all the filters used are only manually and factory adjustable, and are of large size.
By virtue of the controllable frequency filter of the invention, it is possible to implement a circuit which also possesses agility at the level of the frequency synthesis to generates signals Fol1 and, especially, Fol2 : this agility shall then be followed by the agility of the variable frequency filter to deliver a signal at the desired frequency.
Frequency synthesizers agile in the UHF band are now well developed. Indeed, before these synthesizers, quartz oscillators trimmed to a given frequency were used. The frequency of the quartz was then multiplied to arrive up to the UHF transposition frequency, that is comprised between the 470 MHz-860 MHz band minus the intermediate frequency (since it is here the transposition frequency Fol2). However, it was in this case frequency synthesis at fixed frequency.
Nowadays, frequency synthesis has evolved and allows agility at the level of the above transmission systems, except that the second frequency transposition (at frequency Fol2)requires filtering spuriousxe2x80x94and therefore undesirablexe2x80x94band mixtures. This was where agility and integration were lacking.
Thus, by virtue of the invention, there is no longer any need to insert a fixed, factory-calibrated filter 18 in the circuit 10 of FIG. 3: the apparatus is a system that will allow the frequency transposition in a frequency agile manner up to the output to the power amplifier.